ON THE OCCURRENCE OF AN ELECTRON-TRANSFER STEP IN AROMATIC NITRATION

The possibility that aromatic nitration proceeds via the formation of an electron donor-acceptor complex and its possible evolution in a ''c ontact'' radical pair is discussed on the basis of ab-initio configura tion interaction computations on benzene/toluene NO2+ systems. The ana lysis of the region of the potential energy hypersurfaces correspondin g to the two reactants kept at van der Waals distances shows the exist ence of a conical intersection between the ground state and the first excited charge transfer singlet, leading to electron transfer from the aromatic substrate to the nitronium ion, The activated state for elec tron transfer (ET) might then be identified with the ''early'' transit ion state invoked by Olah to explain the high positional selectivity o f substitution in spite of a low substrate selectivity. The retention of positional selectivity at encounter-limited rates may then be ascri bed to the fact that the formation of a Wheland intermediate correspon ds to a radical pair recombination and as such is spin density driven. The objection that ET is a kinetically difficult step is met, the com puted upper limit of the barrier to ET being 13 kcal/mol for the tolue ne substrate.